An example of this type of cut-off device is described in publications FR 2 818 434 and FR 2 891 395 by the same applicant and relates in particular to switches, fuse switches, commutators, reversing switches, circuit breakers or similar appliances.
To establish an electrical current, an electrical device comprises at least a pair of fixed contacts and one moving contact. When closing the electrical circuit, the moving contact is brought against the fixed contact. To ensure the quality of the electrical contact, which is the guarantee for the proper operation of the device, the movement of the moving contact during its approach stroke must be independent of any external action. To that purpose, the device includes a contact closing and opening mechanism, called also actuator mechanism, which consists in an operator-independent snap closing mechanism. During closing, this mechanism gives the moving contact a displacement speed of the order of some m/s. In this period, the moving contact gains a certain quantity of kinetic energy. This results in a shock when the moving contact comes into contact with the fixed contact. The kinetic energy of the moving contact is transformed into material deformation and into an inversion of the speed, resulting in the re-opening of the moving contact, causing a rebound of the moving contact. The re-opening stroke will depend on the approach speed, the nature of the materials, the pressing force maintaining the moving contact against the fixed contact. An electrical arc is generated during this re-opening which, depending on its intensity, will lead to the local melting of the materials of the contact areas. Depending on the conditions, there can be a single rebound or several consecutive rebounds, with damped amplitudes, until the kinetic energy of the moving contact is dissipated. At the end of the rebounds, the electrical contact is established on a molten section that, with time, will solidify. The risk of welding of the moving contact onto the fixed contact is latent. This leads in any case to an erosion of the contact areas that, if it is repeated, may become prejudicial to the normal operation of the electrical device.
One of the solutions to avoid these risks of welding is to differentiate the approach area from the permanent contact area. In the case of sliding contacts, the approach area can correspond to the chamfer lead, as in publications FR 2 524 195, FR 2 638 017 and EP 1 026 710. The making capacity of sliding contacts for short-circuit currents is limited to the power of the actuating mechanism that must oppose the Laplace forces. In addition, the closing of the contacts by sliding takes place while the current is applied, which leads inevitably to an erosion of the permanent contact areas, limiting the endurance of the device. A solution used in the case of pressure contacts is to equip the contact areas with tips, rivets or similar made out of different materials, of which one is generally refractory, in order to limit the risks of welding, or to provide a raised section acting as a spark arrestor, as in publication EP 1 085 610. The implementation of these embodiments is relatively costly.
Publication WO 2006/137687 A1 proposes a cut-off device provided with distinct mobile contacts for phase and neutral, having different lengths in order to generate different strokes, so as to shift the approach of said moving contacts in time. The time shift obtained is higher than 0.1 s, to let relays time to analyze the current in order to detect a possible malfunction. But this arrangement does not allow solving the problem posed.